Improve markdown header spacing
Improve markdown header spacing

Summary:
The spacing above headings didn't look right.

Also workplace purposely limits the different heading sizes to 2 to encourage using notes for large posts. So I'm changing this to match. If we need to have arbitrary depth in future, we can add a `workplaceMode` toggle, but so far that's the only use case we have.

Reviewed By: mweststrate

Differential Revision: D18638861

fbshipit-source-id: 3949a37515e4282bb8435c2740ef45a55e5a97cc

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